Introduction 362Determination of populations and management units 363 Mismatch between management and biological units in the NE Atlantic 371Fisheries management and management units in the NE Atlantic 371Population structure of demersal fish species 372Cod 372Haddock 378Other demersal roundfish species 378Plaice 379Sole 379Other flatfish species 380 AbstractAn essential prerequisite of a sustainable fisheries management is the matching of biologically relevant processes and management action. In fisheries management and assessment, fish stocks are the fundamental biological unit, but the reasoning for the operational management unit is often indistinct and mismatches between the biology and the management action frequently occur. Despite the plethora of population genetic data on marine fishes, to date little or no use is made of the information, despite the fact that the detection of genetic differentiation may indicate reproductively distinct populations. Here, we discuss key aspects of genetic population differentiation in the context of their importance for fisheries management. Furthermore, we evaluate the population structure of all 32 managed marine fish species in the north-east Atlantic and relate this structure to current management units and practice. Although a large number of studies on genetic population structure have been published in the last decades, data are still rare for most exploited species. The mismatch between genetic population structure and the current management units found for six species (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Micromesistius poutassou, Merluccius merluccius and Clupea harengus), emphasizes the need for a revision of these units and questions the appropriateness of current management measures. The implementation of complex and dynamic population structures into novel and less static management procedures should be a primary task for future fisheries management approaches.

Today's Wadden Sea is a heavily human-altered ecosystem. Shaped by natural forces since its origin 7,500 years ago, humans gradually gained dominance in influencing ecosystem structure and functioning. Here, we reconstruct the timeline of human impacts and the history of ecological changes in the Wadden Sea. We then discuss the ecosystem and societal consequences of observed changes, and conclude with management implications. Human influences have intensified and multiplied over time. Large-scale habitat transformation over the last 1,000 years has eliminated diverse terrestrial, freshwater, brackish and marine habitats. Intensive exploitation of everything from oysters to whales has depleted most large predators and habitat-building species since medieval times. In the twentieth century, pollution, eutrophication, species invasions and, presumably, climate change have had marked impacts on the Wadden Sea flora and fauna. Yet habitat loss and overexploitation were the two main causes for the extinction or severe depletion of 144 species ($20% of total macrobiota). The loss of biodiversity, large predators, special habitats, filter and storage capacity, and degradation in water quality have led to a simplification and homogenisation of the food web structure and ecosystem functioning that has affected the Wadden Sea ecosystem and coastal societies alike. Recent conservation efforts have reversed some negative trends by enabling some birds and mammals to recover and by creating new economic options for society. The Wadden Sea history provides a unique long-term perspective on ecological change, new objectives for conservation, 84-95 DOI 10.1007/s10152-004-0209-z restoration and management, and an ecological baseline that allows us to envision a rich, productive and diverse Wadden Sea ecosystem and coastal society.

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About 80 non-indigenous species are assumed to have been introduced into the North Sea by transoceanic shipping and aquaculture. The number is certainly underestimated as most small organisms received insufficient attention at the species level. Also, the seafaring tradition of the North Sea countries is much longer than our biological surveys are. Most exotic invertebrates originate from the western Atlantic and were introduced by shipping, while most algae stem from the Pacific and came with the introduced oysters. A peak of newcomers was observed in the 1970s. Most of the arrivals became established in brackish environments, at harbor sites and in the vicinity of oyster farms, fouling on hard substrates or living as epibionts. A few live in sediments, are holoplanktonic or are parasites. At the open coast, approximately 6% of the macrobenthic species are exotics, while in estuaries their share is up to 20%. Most exotics have been encountered in the southern North Sea first, and many did not spread further north. About 25% of the established non-natives are widespread and attain locally high abundances. As a consequence, some inshore habitats are entirely dominated by exotics. The overall effect on the ecosystem seems to be more additive than one of displacement. This suggests that the coastal biota of the North Sea are quite capable of accommodating newcomers. However, this is no guarantee that the next introduced species may not cause severe ecological change or economic harm. There is a need to minimize the risk of unintentional introductions by ballast water treatment and by adhering to quarantine procedures in aquaculture. Current research on exotics in the North Sea is regarded as inadequate for proper evaluation and management requirements. I N T R O D U C T I O NM a r i t i m e traffic across the o c e a n s a n d c u l t u r i n g of n o n m a t i v e o r g a n i s m s at the e d g e of the s e a c o n t r i b u t e d to the s p r e a d a n d e s t a b l i s h m e n t of a n e v e r i n c r e a s i n g n u m b e r of exotic s p e c i e s in coastal a n d b r a c k i s h w a t e r e n v i r o n m e n t s . Exotic species of the N o r t h Sea are h e r e o p e r a t i o n a l l y d e f i n e d as s p e c i e s o c c u r r i n g only o u t s i d e the r e g i o n of t h e A t l a ntic c o a s t of E u r o p e (Gibraltar to N o r t h C a p e ) ; h e n c e M e d i t e r r a n e a n a n d P o n t o -C a s p i a n s p e c i e s are c o n s i d e r e d as exotics. We f u r t h e r restrict this o v e r v i e w to s p e c i e s w h i c h h a v e p r e s u m a b l y a r r i v e d by m e a n s of h u m a n t r a n s p o r t , b o t h i n t e n t i o n a l as w e l l as u n i n t e ntional i n t r o d u c t i o n s . B o u n d a r i e s of the N o r t h Sea are d e f i n e d a c c o r d i n g to t h e N o r t h S e a T a s k Force (1993), w h i c h i n c l u d e s t h e C h a n n e l r e g i o n in t h e south, t h e S k a g e r r a k a n d K a t t e g a t in t h e east, a n d t h e S h e t l a n d I s ...

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a b s t r a c tThe Wadden Sea is one of the largest intertidal areas in the world and has been designated as a UNESCO World Heritage Site in recognition of its unique natural features. Major changes in the morphology and ecology of the Wadden Sea over the past millennium resulted from increasing anthropogenic influences such as coastal protection, land claim from the sea and drainage of wetland for agriculture, exploitation of natural resources from hunting and fishing to the extraction of groundwater, gas and oil, industrialisation at port locations and tourism at the islands. A sustainable future can only be achieved if policy and management are backed by solid science. Many of the anticipated changes result from the upscaling of pressures on the Wadden Sea system. Economic globalization leads to upscaling of fisheries, tourism and industrial activities, and thus to changed pressures on space and nature. Climate change will lead to changes in hydrographic patterns, species distribution and possibly tourism; through sea-level rise it will put pressure on coastal protection and the extent of intertidal areas. Invasions of exotic species will transform the ecosystem. There are three major related challenges to management: 1. Nature conservation in a changing system requires a focus on preservation of the values and not the state of the system; 2. The adaptation of the management structure to the scale increase of the pressures, so that local and regional management becomes better nested in transregional and transnational governance structures; 3. Finally, the management approach needs to deal with increasing uncertainty in external forcing of the system, as well as with nonlinearities in system dynamics when it is pushed outside its normal range of operation. Based on these pressures and management challenges, we advocate an integrated social-ecological systems approach for the scientific study and the science-based management of the Wadden Sea Region. The essential characteristics of this approach are strong interdisciplinarity and a focus on aspects of scale and cumulative processes.

The numbers of exotic species introduced into brackish waters (5-20 psu) and highsalinity waters (> 20 psu) in the Netherlands are hypothesized to reflect species richness in such waters elsewhere in the world. Notwithstanding the fact that species numbers m brackish waters all over the world are lower than in high-salinity waters, the numbers of introduced species in these waters in the Netherlands are about equal. Alternative hypotheses to explain this phenomenon are: (1) because most ports are situated in brackish regions, brackish-water species stand a better chance of being transported; (2) because brackish-water species are more tolerant of conditions in ballast water tanks, these species have a better chance of being transported alive than high-salinity species; and (3) because brackish waters have few species, it is easier for an introduced species to estabhsh itself in brackish waters. None of the latter three hypotheses can be rejected and probably they all play a part in explaining the phenomenon. The third hypothesis, however, seems most likely. I N T R O D U C T I O NT h e w e l l -k n o w n r e l a t i o n s h i p b e t w e e n salinity a n d s p e c i e s n u m b e r s w a s first d e m o n s t r a t e d b y R e m a n e (1934, 1971). H i g h s p e c i e s n u m b e r s o c c u r b o t h in f r e s h w a t e r a n d s e a w a t e r , w h e r e a s b r a c k i s h w a t e r s are c h a r a c t e r i z e d b y a low n u m b e r of species, s o m e of w h i c h are c h a r a c t e r i s t i c of b r a c k i s h w a t e r s . A l t h o u g h originally t h e r e l a t i o n s h i p s h o w n b y R e m a n e w a s b a s e d o n d a t a f r o m t h e Baltic a n d o t h e r r e g i o n s of n o r t h e r n E urope, t h e r e l a t i o n s h i p also a p p e a r s to h o l dfor m a n y o t h e r p a r t s of t h e world, e.g. t h e B l a c k S e a ( R e m a n e , 1971), t h e A t l a n t i c coast of N o r t h A m e r i c a (Boesch, 1972), a n d S o u t h Africa (Day, 1981). I a s s u m e t h a t l o w s p e c i e s n u m b e r s are a f e a t u r e of b r a c k i s h w a t e r s all o v e r t h e world.In r e l a t i o n to t h e i n t r o d u c t i o n of exotic s p e c i e s to o t h e r parts of t h e w o r l d it m i g h t b e a s s u m e d as a first a p p r o x i m a t i o n t h a t b r a c k i s h -w a t e r s p e c i e s h a v e t h e s a m e c h a n c e of b e i n g t r a n s p o r t e d as h a v e h i g h -s a l i n i t y s p e c i e s or f r e s h w a t e r species. H e n c e , I form u l a t e as a n h y p o t h e s i s to b e t e s t e d t h a t t h e n u m b e r of exotic s p e c i e s i n t r o d u c e d into b r a c k i s h w a t e r s is c o n s i d e r a b l y less t h a n t h e n u m b e r of s u c h s p e c i e s i n t r o d u c e d into h i g h -s a l i n i t y c o a s t a l w a t e r s .As b r a c k i s h -w a t e r s p e c i e s I c o n s i d e r t h o s e i n h a b i t i n g p a r t s of e s t u a r i e s a n d l a g o o n s w i t h a salinity n o r m a l l y b e t w e e ...

About 80 non-indigenous species are assumed to have been introduced into the North Sea by transoceanic shipping and aquaculture. The number is certainly underestimated as most small organisms received insufficient attention at the species level. Also, the seafaring tradition of the North Sea countries is much longer than our biological surveys are. Most exotic invertebrates originate from the western Atlantic and were introduced by shipping, while most algae stem from the Pacific and came with the introduced oysters. A peak of newcomers was observed in the 1970s. Most of the arrivals became established in brackish environments, at harbor sites and in the vicinity of oyster farms, fouling on hard substrates or living as epibionts. A few live in sediments, are holoplanktonic or are parasites. At the open coast, approximately 6% of the macrobenthic species are exotics, while in estuaries their share is up to 20%. Most exotics have been encountered in the southern North Sea first, and many did not spread further north. About 25% of the established non-natives are widespread and attain locally high abundances. As a consequence, some inshore habitats are entirely dominated by exotics. The overall effect on the ecosystem seems to be more additive than one of displacement. This suggests that the coastal biota of the North Sea are quite capable of accommodating newcomers. However, this is no guarantee that the next introduced species may not cause severe ecological change or economic harm. There is a need to minimize the risk of unintentional introductions by ballast water treatment and by adhering to quarantine procedures in aquaculture. Current research on exotics in the North Sea is regarded as inadequate for proper evaluation and management requirements. I N T R O D U C T I O NM a r i t i m e traffic across the o c e a n s a n d c u l t u r i n g of n o n m a t i v e o r g a n i s m s at the e d g e of the s e a c o n t r i b u t e d to the s p r e a d a n d e s t a b l i s h m e n t of a n e v e r i n c r e a s i n g n u m b e r of exotic s p e c i e s in coastal a n d b r a c k i s h w a t e r e n v i r o n m e n t s . Exotic species of the N o r t h Sea are h e r e o p e r a t i o n a l l y d e f i n e d as s p e c i e s o c c u r r i n g only o u t s i d e the r e g i o n of t h e A t l a ntic c o a s t of E u r o p e (Gibraltar to N o r t h C a p e ) ; h e n c e M e d i t e r r a n e a n a n d P o n t o -C a s p i a n s p e c i e s are c o n s i d e r e d as exotics. We f u r t h e r restrict this o v e r v i e w to s p e c i e s w h i c h h a v e p r e s u m a b l y a r r i v e d by m e a n s of h u m a n t r a n s p o r t , b o t h i n t e n t i o n a l as w e l l as u n i n t e ntional i n t r o d u c t i o n s . B o u n d a r i e s of the N o r t h Sea are d e f i n e d a c c o r d i n g to t h e N o r t h S e a T a s k Force (1993), w h i c h i n c l u d e s t h e C h a n n e l r e g i o n in t h e south, t h e S k a g e r r a k a n d K a t t e g a t in t h e east, a n d t h e S h e t l a n d I s ...